Electromagnet



J. KELAR ELECTROIIAGNET July 18,- 1950 Filed Jan. 24, 1946 2 Sheets-Sheet 1 INVENTOR Jafgnb /fldr BY ATTORNEY July 18 1950 l J. KELAR 2,515,305

ELEcTRouAGN-ET Filed Jan. 24, 194e 2 sheets-sheet 2 MMF/gp l j; @y

L? I In INVENTOR JaxpLJclar ATTORNEY Patented July 18, 1950 ELECTROMAGNET Joseph Kelar, Lancaster, Pa., assigner to Radio Corporation of America, a corporation of Dela- Application January 24, 1946, Serial No. 643,095

8 Claims. (Cl. 175-21) The present invention is for use with electronic tubes of the type in which a cathode ray beam is developed and then deflected to eilect the reproduction of a desired condition, and more particularly relates to a method and means for maintaining the axial alignment of the electron scanning beam of cathode ray tubes which include an ion-segregating device.

It has been a characteristic of magnetically defiected cathode ray tubes as used in television and oscillographic systems that a so-called ion spot is formed on the screen of the tube after a period of use. This is due to the fact that the relatively heavier ions in the developed cathode ray beam are not as readily deflected by a magnetic field as are the relatively lighter electrons, and, therefore, tend to continue approximately along the axis of the tube thereby to cause a steady bombardment of the central portion of the fluorescent screen. In time this brings about a permanent darkening or discoloration of the central portion of the screen.

In a copending U. S, patent application of this applicant, Serial No. 726,589, filed February 5, 1947, now Patent No. 2,496,127, means have been disclosed whereby the ions and other heavy particles contained in the developed cathode ray beam may be substantially eliminated from the beam before it strikes the screen of the cathode ray tube by the use of a particular construction for the electron gun of the tube. Since it is well known in the field of electron optics that the first and second anodes of the cathode ray tube together constitute an electron lens which may be eiiective to alter the path of the developed beam, use has been made of this principle in designing a lens the plane of which is not normal to the axis of the tube, but is, instead, oblique thereto. As a consequence of the above, both the ions and the electrons which together make up the developed cathode ray beam are caused to deviate from the axis of the gun toward the axis of the obliquely positioned, or tilted, electron lens. Normally, the result of such a construction would be that both the ions and electrons in the beam would impinge on the walls of the gun or on a subsequent aperture disk therein.

By this invention, however, there is superimposed on the electrostatic ileld of the electron lens a magnetic field of such direction and magnitude that the electrons in the beam are caused to return substantially to the axis of the gun, while the heavy particles and ions, which are heavy or heavier than the electrons and hence not deiiected to the same degree by a magnetic eld asl are the electrons, continue on the path to which they were diverted by the tilted electron lens until they are collected by some portion o! the gun structure in the manner indicated above. The beam which strikes the tube screen, therefore, is relatively free of ions, and hence no darkening or discoloration of the fluorescent coating occurs.

It has been found, however, that when a magnetic iield is superimposed on the electrostatic field of the electron lens in the manner above set forth, the electron beam does not leave the gun along a path coincident with the axis of the tube. This is due to the fact that while the bend, or change in direction, of the electron beam caused by the magnetic eld substantially compensates for the bend, or change in direction, of the electron beam caused by the electrostatic field of the electron lens, nevertheless the doubly deflected beam crosses the axis of the tube at an angle thereto, or is other than coincident with the axis of the tube neck.

According to a feature of the present invention, the magnetic iield referred to above is deliberately overstrengthened to insure the beam crossing the tube axis at an angle thereto. A second magnetic eld is superimposed on the electron beam approximately at the point where the doubly deflected beam crosses the tube axis. This second magnetic field is substantially parallel to the first magnetic eld but is of opposite polarity thereto. By properly relating the strength of the two elds, the axis along which the electron beam travels is brought into substantial coincidence with the axis of the cathode ray tube.

One object of the present invention, therefore, is to provide a, method of, and apparatus for. axially aligning the electron beam in a cathode ray tube of the type which includes an ion-segregating or trapping device.

Another object of the present invention is to provide, for a cathode ray tube of the type in which the developed cathode ray beam is electrostatically focused at a point other than along the axis of the cathode ray tube, a method and means for magnetically centering the electron beam within the tube neck.

A further object of the invention is to provide for the dual bending of a cathode ray beam which has been electrostatically diverted from the axis ofa cathode ray tube.

A still further object of the invention is to provide for the dual bending of a cathode ray beam which has been subjected to the divertiva innuen of an electrostatic seid, said dum bend,-

ing res Tting from the action of two parallel mag'- netic elds of opposite polarity acting substantially transverse to the axis along which the cathode ray beam is projected.

An additional object is to provide an improved type of electromagnet assembly for producing such parallel magnetic elds.

Other objectsand advantages will be apparent from the following description of a preferred form of the invention and from the drawings, in which:

Figure 1 is a sectional view of a portion of a cathode ray tube incorporating an ion-segregating device, and illustrating in purely schematic manner the principle of the present invention;

Fig. 2 is a view of a portion of Fig. 1, showing in greater detail the angular deviations of the' electron beam caused by twoA magnetic elds generally designated A and B as td the region of effectiveness although not as to the direction of eect.

Fig. 3 is a perspective view of a preferred form of electrom'agnet assembly in accordance with the present invention; and

Fig. 4 is a side view of the electromagnet assembly of Fig. 3, showing one manner of energizing the coils.

Referring ilrst to Fig. 1, there is shown a cathode ray tube generally indicated by the reference character I0. Tube l is formed with a neck portion l2 and a bulb portion I4. The neck portion l2 includes an electron gun comprising a unipotential cathode I6, a heater fllament I8, and a control electrode or grid 20. Also included in the electron gun structure is a cylindrical rst anode 22 and a cylindrical second anode 24. The showing of the exact electrode structure is somewhat schematic and is set forth in more detail in the above-mentioned companion application.

Connected to the cylindrical second anode 24 to form in eifect an electrical extension thereof is a conducting wall coating 26, of a substance such for example as aquadag, which covers the inner surface of the bulb portion I4 of tube I0 and extends into the neck portion l2 as shown. A ilexible connection such as a spring element 2t is secured at oneend to the second anode 24, the other end of the spring element 28 being in frictional engagement with the conducting wall coating 26. The coating 26, and hence the cylindrical second anode 24, is maintained at a desired potential diierence relative tothe rst anode 22 by means of a terminal lead 30 extending through the wall of bulb portion I4, as illustrated.

As an alternative construction to that set forth above, the spring element 28 may be eliminated, and the second anode 24 connected to a separate source of potential which is positive with respect to the rst anode 22. If desired, means may also be provided for varying the potential diiierence between anodes 22 and 24.

Bulb portion I4 is provided with the usual electron beam responsive target such as a luminescent or fluorescent screen of any suitable composition (not shown). Encircling the neck portion l2 of tube Il! is a deflecting yoke 32 including a pair of horizontal, or line, deection coils, and a pair of vertical, or frame, deecting coils. Since these coils form no part of the present invention, the yoke 32 is shown more or less schematically in order to simplify the drawing. The system is not to be restricted to use with magnetic deection in both horizontal and vertiection in one direction, and electrostatic deection in the other (as in certain radar tubes) with benecial reduction ion discoloration. (Tn

this case the ion discoloration would normally be a line instead of a spot.)

The customary potential may be applied to the various components of the electron gun assemblyV of tube lil, and hence the source of these potentials has not been illustrated. As previously stated-however, a suitable potential difference exists between the ilrst anode 22 and the cylindrical second anode 26, and, as a result of this potential dierence and the spacing between these elements, an electrostatic lens 3d is formed.

While connections to voltage sources are not shown (for simplicity of illustration), these are Well @own and any suitable voltage values may be used although reference :may bernade in this connection to George U. S. Patent No. 2,086,546, granted July 13, 1937, and the Chevallier U. S. Patent No. 2,021,252, granted November 19, 1935.

Further, since the general principles underlying the formation of the electrostatic lens 36 are known in the art no further explanation will be set forth herein but additional reference maybe made to the publication entitled Electron Optics in Television, by I. G. Maloff and D. W. Epstein (McGraw-Hill Book Company), for a detailed discussion of the principles involved.

The adjacent surfaces of the first anode 22 and the cylindrical second anode 2li, which, by reason of the potential diilerence and the spacing therebetween constitute the electrostatic lens Sil, are so arranged relative to one another that the planes of the surfaces are substantially parallel. Furthermore, these planes are disposed to form an angle other than with the axis of the anodes 22 and 2d, and hence with the axis of the neck portion I2 of tube lli. Consequently, the electrostatic lens 36 is in eect tilted as shown in Fig. l, so that its axis does not coincide with the axis of the cathode ray tube.

The developed cathode ray -beam which is illustrated schematically and designated by the reference character 3% in Fig. 1, is diverted by the lens 34 from its normal path along the axis of the cylindrical anodes 22 and 2d (which is also the axis of tube It) to take a path which may be such as indicated by the reference character 38. Normally this diverted cathode ray beam, including both ions and electrons, would strike the wall of anode 254 at some point such as 4t.

Now, however, there is superimposed on the deviated cathode ray beam following the path 38 arst magnetic field A which is substantially transverse to the axis of the cathode ray tube l0, andsimultaneously transverse to the desired plane direction of beam bending. The direction of this first magnetic field A is such as to deviate, or bend, the electrons in the beam in a direction opposite to the deviationproduced as a result of the action of the electrostatic lens 34. Consequently, the electrons in the beam are caused to follovs7 a new path 62, as shown in Fig. 1. The ions and other heavy particles, however, are not iniluenced by a magnetic eld to the same extent as are the lighter electrons, and hence continue along the path 38 until they strike the cylindrical second anode 24 approximately at the point 40.

The electrons which are caused to follow the path 42 after passing through the rst magnetic eld A are traveling at an angle to the axis i tube I0. If they were allowed to continue in such a direction, they would either strike the screen of the tube i0 a a point not in the center thereof, or else they would not pass through the aperture disk 44, depending on the magnitude of the angle made by the electrons with the tube axis. To overcome this condition, a second magnetic field B is provided. This second magnetic field B is substantially parallel to the first magnetic field A, and hence is also transverse to the axis o! tube l0. The second magnetic eld B, however, is of a polarity opposite to that of the ilrst magnetic eld A, and, as a result, the former causes the electron beam to be bent or deviated back along a path 46 which is substantially co-linear with the aXis of the cathode ray tube.

The aligned electron beam following path 46 is the one which actually strikes the screen of tube i0, and since the ions have been separated from the beam as a result of their collection by the anode 24 at or near point 40, no ion spot or othery undesirable effect as a result of the presence of ions in the original electron scanning beam is produced.

Fig. 2 illustrates in greater detail a portion oi Fig. l. especially as regards the action of the two magnetic elds.- The developed cathode ray beam 36 is caused to follow a path such as 38 by the action ofthe electrostatic lens 34. The ilrst magnetic eld A bends the electrons in the beam through an angle e, so that the electrons are caused to follow a path 42 which is at an angle d to the axis 48 of tube ill. Accordingly, to align the electron beam with the tube axis, the second magnetic iield B must bend the electron beam through the angle rp. It has been found in practice that, to bring about the desired action, the iirst magnetic field should normally be greater in intensity than the second magnetic field.

In Figs. 3 and 4 there is illustrated one preferred form of electromagnet assembly for producing the two magnetic fields referred to in connection with Figs. l and 2. This electromagnet assembly ci" Figs. 3 and i is generally indicated by the reference character D, and is designed to be placed around the neck portion i2 of tube i0 of Fig. 1 in such a manner that the elds A and B produced thereby (the directions of which are indicated by the arrows in Fig. 3) have respective positions relative to the electrostatic lens 34 such that the force exerted on the electrons in the cathode ray beam is in the direction indicated by the arrows in Figures l and Z.

As shown in Fig. 3, the electromagnet 50 has two pairs of pole pieces 52 and 5G respectively. On each of the legs of the magnet which includes the pole pieces 52 is wound a coil These coils 5t preferably have an equal number of turns and are otherwise substantially identical. Hence, by connecting the coils in series with each other and with a source or direct current as shown in Fig. a relatively constant magnetic eld A is produced in the space between the pole pieces 52, as shown in Fig. 3. Although the intensity of this field A may be selected in accordance with the operating conditions encountered, it has been found that, for a magnetically focused cathode ray tube operated at 6 kilovolts, the coils 56 should preferably be constructed to have in the order of 25) ampere turns.

Respectively extending from the opposite ends of the coils 56 to the pole pieces 52 is a pair of core segments 58. These core segments 58 are designed to lie in substantially coplanar relation, as

shown in Figs. 3 and 4. The ends of the core segments 5l are spaced apart to form an air gap 60.

' The pair of pole pieces 54 lie in a plane substantially parallel to the plane formed by the pole pieces 52, so that the magnetic field B produced in the space between the pole pieces 54 is substantially parallel to the magnetic field A produced in the space between the pole pieces l2. The pole pieces 54, however, are secured respectively to the core segments 55, so that the polarity of the ield B is opposite to that of the field A, as shown by the arrows in Fig. 3.

It will be noted that the pole pieces 54 are effectively in shunt with the core segments 5l, or, in other words, the magnetic iield B may be said to be in parallel relation with the magnetic field produced in the air gap 60 between the core segments 58. Therefore, a relationship exists between these two elds, and a change in one iield will produce a change in the other. By increasing the width of the air gap 60, the magnetic eld B may be increased in intensity, and, inversely, a decrease in the width of the air gap 60 will increase the magnetic flux density in the core segments 58 and thus decrease the intensity of the magnetic field B between the pole pieces 5t. By varying the width of the air gap 8l, therefore, the intensity ratio of the magnetic fields A and B may be adjusted in any desired manner to effect an axial alignment of the electron scanning beam of tube It.

A pair of openings S2 respectively formed in the core segments 58 permit the electromagnet assembly 50 to be bolted or otherwise secured to a suitable supporting member. This supporting member may be of any appropriate type, and hence has not been illustrated in order to simplify the drawings.

Having thus described my invention, I claim:

l. An electromagnet assembly comprising a pair or coils. a pair o core sections respectively passing through said pair of coils, one end of each core section of said pair being so designed that the two ends together form a pair of pole pieces for said assembly, the remaining ends of said pair of core sections being .arranged to extend toward one another in such a manner as to es tablish an air gap therebetween, and a second pair oi' pole pieces for said assembly. said second pair of pole pieces being respectively secured to the said remaining ends of said pair of core sections, and disposed to lie in a plane substantially parallel to the plane of the pole pieces formed by said one end oi'; each core section of said pair.

2. An electromagnet assembly according to claim l, in which said iirst Aand second pairs of pole pieces are disposed so as to lie sidebyside in spaced-apart relationship.

3. An electromagnet assembly comprising a core having two oppositely-disposed legs, a pair of coils respectively wound on the said legs, one end of one of said legs forming, with the cor responding end of the other of said legs, a pair oi' pole pieces, the remaining ends oi said legs extending toward one another so as to create an yair gap therebetween, and a pair of supplemental core sections respectively secured to the said remaining ends of said legs, whereby said pair of supplemental core sections form in eiect a second pair of pole pieces, and whereby the magnetic field established between said second pair of pole pieces upon a constant selective energization oi said coils has an intensity determined in part by the dimensions of said air gap.

4. An electromagnet assembly comprising a core having two oppositely-disposed legs, Va pair of coils respectively wound on the said legs, one end oi' one of said legs being designed to form, withl one end of the other of said legs, a pair/of pole pieces, the remaining ends of said legs extending toward one another so as to create an air gap therebetween, and a pair of auxiliary core sections respectively secured to the said remaining ends of said legs, said pair of auxiliary core sections having portions lying in a plane substantially parallel to the plane formed by said pair oi pole pieces, so that said pair of pole pieces and said portions of said pair of auxiliary core sections 'are disposedv side-by-side in spaced-apart relation, and so that upon a substantially constant energization of' said coils the magnetic field produced between said pair of pole pieces will be of opposite polarity to the magnetic field produced between said Aportions of said pair of auxiliary core sections. s

5. An electromagnet including a core, a coil surrounding a portion of said core, saidl core comprising a pair of spaced pole pieces between which there is produced a first magnetic field upon energization of said coil, and means forming an extension of said core for producing a second magnetic field o f opposite polarity to said first-mentioned magnetic field, said extension means being so positioned relative to said pair of pole pieces that the said first and second magnetic fields are efiectively in parallel relationship.`

6. An electromagnet comprising a core, a coil surrounding a portion of said core, said core including a pair of spaced pole pieces between which pieces that the two said magnetic fields are effectively in parallel relationship, said core formed with an air gap therein for determining the intensity of said second magnetic field.

1. An electromagnet comprising a core, a coil surrounding a portion of said core, said core including a pair of spaced pole pieces between which there is produced a first magnetic field upon en ergization of said electromagnet and means forming an extension of said core for producing a second magnetic field of opposite polarityto said first-mentioned magnetic field, said extension means being so positioned relative to said pair of pole pieces that the two said magnetic fields are substantially in parallell relationship, said core including a pair of spaced portions forming an air gap therebetween to determine the intensity of said second magnetic field, said spaced portions having means toadjustably mount said portions in spaced relationship for Varying the air gap therebetween.

8. An electromagnet assembly comprising a pair of coils, a core section passing through each coil, one end of each core section positioned and arranged toform one of a pair of spaced pole pieces for.said assembly, the other` ends of said core sections arranged to extend toward one another and form an air gap therebetween, and a second pair of pole pieces, one of said second pair of pole pieces respectively secured to each of said other core section ends, and means to adjustably mount said core sections in spaced relationship for varying said all' gap therebetween.

JOSEPH KELAR.

REFERENCES CITED The following references are of record in the tile of this patent:

UNITED STATES PATENTS Great Britain Feb. 21, 1940 

